HMF has been recognized as a chemical with potentially significant industrial and commercial applications because of its high degree of functionality and its ability to act as a precursor to various industrially useful chemicals. See Werpy, T; Petersen, G. (Eds.), “Top Value Added Chemicals from Biomass, Vol. 1: Results of Screening for Potential Candidates from Sugars and Synthesis Gas,” U.S. Dept. of Energy, Office of Scientific Information: Oak Ridge, Tenn. DOE/GO-102004-1992 (2004). For example, its functionality affords use in the production of solvents, surfactants, pharmaceuticals and plant protecting agents, and furan derivatives thereof which are useful as monomers for the preparation of non-petroleum derived polymers.
HMF is primarily produced by dehydrating a carbohydrate feedstock, particularly monosaccharides such as glucose and fructose. Complications commonly arise during the reaction as a result of the production of unwanted acid by-products, particularly levulinic and formic acid, and especially the polymerization of reaction components which forms humins (a mixture of colored, soluble and insoluble oligomers and polymers), all of which reduce the overall process yield and complicate the recovery of HMF, making large scale production of HMF economically unattractive. These complications are exacerbated by the desire to maximize conversion of feedstock to HMF in the reaction zone.
Fructose is the preferred hexose to produce HMF because it has been demonstrated to be more amenable to dehydration reactions than other hexoses including glucose. High fructose corn syrup (HFCS) is a high volume, commercially available product from which HMF and other furans could be produced in large quantities. Currently, as much as 18 billion pounds/yr of high fructose corn syrup are produced. Szmant et al, J. Chem. Tech. Biotechnology, Vol. 31, PP 135-45 (1981) disclosed the use of high fructose corn syrup as a feedstock for the production of HMF.
A variety of homogeneous catalysts have been employed to promote the dehydration of fructose to HMF. Inexpensive strong inorganic acids have been used: see, for example, U.S. Pat. No. 7,572,925. Organic acids have also been disclosed, including relatively strong organic acids such as p-toluene sulfonic acid and weaker organic acids such as oxalic acid and levulinic acid: See, for example, U.S. Pat. No. 4,740,605, which discloses oxalic acid. All patents and other publications cited in this application are incorporated herein by reference.
Similarly, a variety of heterogeneous catalysts have been reported as useful for the dehydration of carbohydrate to HMF. See, for example de Vries, Chem. Rev. 2013, pp 1499-1597. Dumesic, ACS Catal 2012, 2, pp 1865-1876; and Sandborn, U.S. Pat. No. 8,058,458. Fleche, in U.S. Pat. No. 4,339,387, disclosed the use of solid acid resin catalysts where the resin may be a strong or weak cationic exchanger, with the functionalization preferably being in the H+ form (including, for example, resins under the trademark Amberlite C200 from Rohm & Haas Corporation and Lewatit SPC 108 from Bayer AG). Sanborn, in AU 2011205116, disclosed that metals such as Zn, Al, Cr, Ti, Th, Zr and V are useful as catalysts. And Binder, in US 2010/0004437 A1, disclosed the use of a halide salt.
In addition to the use of catalysts in the dehydration of carbohydrates to HMF, there has been much focus on solvents and solvent systems that reportedly are beneficial in the process. See for example, de Vries Chem. Rev 2013, 113, 1499-1597.
A multitude of processes have been disclosed for the production of HMF from fructose. However, the known prior processes have not recognized any benefit associated with low conversion in the reaction zone. Typically, research has focused on attaining the highest possible conversion of fructose to HMF in the reaction zone, which inevitably has resulted in increased off-path products, including humins, and/or process complexity and expense. In the quest to attain high conversion of fructose to HMF in the reaction zone, prior processes have focused on improving catalyst performance, reactor solvent systems and reactant mixing techniques, using solvent modifiers to improve phase separations in the reactor, using foam and/or oxidation suppressants, reducing carbohydrate concentration in the reactor, using very high temperatures and/or pressures, and performing multiple steps in the reactor (e.g., steam injection or controlled vaporization to simultaneously remove certain constituents), among other techniques. Nevertheless, none of the processes disclosed to date appears to have overcome the low overall process productivity in a commercially economically viable manner.
In order to overcome the shortcomings of the prior processes, applicants have discovered processes based upon intentionally limiting the conversion of fructose to HMF in the reaction zone. In these processes, HMF, unconverted fructose, solvent and, when applicable, catalyst are removed from the reaction zone and ultimately separated from one another, enabling the efficient recycling of these separated constituents and, ultimately, the cost effective production and recovery of large quantities of HMF.